In both consumer and professional audio processing devices, the use of digital audio signals is increasing rapidly. Increasingly, these audio signals are combined with video signals and may in fact be embedded within a video signal. For example, the ANSI/SMPTE272M-1994 standard sets out a system for embedding audio information in the ancillary data space of a digital video signal. This standard permits up to four audio data packets, which may contain audio data for one or more audio samples, to be transmitted in the ancillary data space for most television lines in each frame of a digital video signal. Some television lines may be limited to carrying less than four samples and some lines may not be used for audio data packets at all.
A typical digital audio/video receiver (e.g. a digital television set) must be capable of receiving an input stream of digital video data with embedded audio and must be able to handle a change in the content of its input stream. Such a change may occur when an upstream device such as a router switches the input stream from one program signal to a second program signal. The two program signals will normally not be synchronized and the switch may result in a period of instability in the input stream. The resulting lack of stability does not substantially affect the processing of the video component of the signal, but it can have a significant effect on the processing of the audio component. Differences in audio level and balancing can result in an audible audio “pop” if the transition is made without any protective circuitry. In some cases, this pop can damage speakers and other circuits, depending on its frequency components and their intensity.
One known solution to this problem is to detect the transition in the input stream and then mute the audio output from the device for a fixed muting period. This approach has several disadvantages. First, the muting period must (or at least should) be selected to be longer than the longest expected period of instability on the input stream. Since this will typically be considerably longer than the average period of instability, the audio component of an audio/video presentation from the receiver will be interrupted for a longer period than is required. Second, many valid audio samples will be discarded during most transitions since the period of instability will be over well before the muting period. Finally, in a case where the period of instability is longer than expected, the muting period may end while the input stream is still unstable. This may result in an audio pop being undesirably transmitted to other devices. In general, this solution is a “brute force” approach to the problem and is relatively inelegant.
Accordingly, there is a need for a circuit for efficiently receiving an input stream of digital video data in which audio data has been embedded and which may have an upstream source transition. The circuit will preferably make use of all valid audio samples available in the input stream and will interrupt the presentation of the audio data for a relatively short period. Preferably, the interruption is made without sudden audio level changes.